Added formatter to column object

[gfoidl]

The table data items are displayed by simple toString (internally).

This PR adds the optional property formatter, so you can do something like

function formatTimeStamp(value: Date): string {
    return value.toLocaleString();
}

function formatNumber(value: number): string {
    return value.toLocaleString(undefined, { maximumFractionDigits: 2, minimumFractionDigits: 1 });
}

const columns = [
    { key: "timeStampUtc", label: "Timestamp", formatter: formatTimeStamp},
    { key: "mean"        , label: "Mean"     , formatter: formatNumber   },
    { key: "median"      , label: "Median"   , formatter: formatNumber   },
    { key: "sigma"       , label: "Sigma"    , formatter: formatNumber   }
];

in order to specify a formatter, that will convert each item's property according the given formatter-function.

Note

It's optional to specify the formatter

[gfoidl]

I know that there may be better ways, but that would need quite a bit of code-change.
This was tested with a rather big table, and no perf-problems did occur, so I'm fine with this approach.